Tidying could cause ill-kinded types
I found (Trac #14175) that the tidying we do when reporting
type error messages could cause a well-kinded type to become
ill-kinded. Reason: we initialised the tidy-env with a
completely un-zonked TidyEnv accumulated in the TcLclEnv
as we come across lexical type-varialbe bindings.
Solution: zonk them.
But I ended up refactoring a bit:
* Get rid of tcl_tidy :: TidyEnv altogether
* Instead use tcl_bndrs :: TcBinderStack
This used to contain only Ids, but now I've added those
lexically scoped TyVars too.
* Change names:
TcIdBinderStack -> TcBinderStack
TcIdBinder -> TcBinder
extendTcIdBndrs -> extendTcBinderStack
* Now tcInitTidyEnv can grab those TyVars from the
tcl_bndrs, zonk, and tidy them.
The only annoyance is that I had to add TcEnv.hs-boot, to
break the recursion between the zonking code and the
TrRnMonad functions like addErrTc that call tcInitTidyEnv.
Tiresome, but in fact that file existed already.